1. Field of the Invention
The present invention relates to a semiconductor wafer in which a silicon thin film is formed over the surface of a transparent insulated board, like SOS (Silicon on Sapphire) or the like, and a method of manufacturing the semiconductor wafer.
2. Description of the Related Art
In a wafer process for manufacture of a semiconductor device, a positioning orientation flat is previously provided at one point on the circumference of a disk type semiconductor wafer. A stage with the semiconductor wafer mounted thereon is rotated and the orientation flat is detected by a photosensor or optical sensor using visible light (e.g., laser light having a wavelength of 633 nm), thereby performing accurate positioning of the semiconductor wafer. In the case of a silicon wafer using a general silicon substrate, the thickness of the silicon substrate is greater than or equal to a predetermined thickness. Therefore, an orientation flat and spots other than it are different in light transmittance and hence the positioning of the silicon wafer can be performed easily.
Each of SOS and SOQ (Silicon on Quartz) processes has recently been used wherein a silicon thin film is formed over the surface of an insulated board of sapphire or quartz or the like, and an integrated circuit is formed over the silicon thin film. Since SOS and SOQ make use of the insulated board respectively, no leak current flows in a substrate like the silicon substrate and a reduction in power consumption is enabled. Therefore, each of SOS and SOQ has come to attention as a semiconductor device built in a portable type device in particular.
Since, however, the sapphire and quartz are high in light transmittance and a silicon thin film formed over its surface is also high in light transmittance because it becomes a monocrystal having a thickness of 1 μm or less, for example. Thus, it is not possible to detect an orientation flat by using the conventional optical sensor in a state of being kept intact.
Therefore, a patent document 1 (Japanese Unexamined Patent Publication No. Hei 11(1999)-220114) discloses a method of manufacturing a semiconductor device, wherein a silicon thin film for formation of circuit elements is formed over the surface of a sapphire substrate, and a light-penetration preventing polysilicon thick film is formed over its back, respectively, and an orientation flat is provided at the entire surface of the polysilicon thick film on the back, thereby preventing warpage and cracks of the substrate.
Further, a patent document 2 (Japanese Unexamined Patent Publication No. 2000-36585) discloses a method of manufacturing a semiconductor device, wherein in order to prevent warpage and cracks of a substrate due to a silicon thick film, a silicon thin film is formed on the obverse and reverse sides of a sapphire substrate, and silicon ions are implanted in the silicon thin film placed over the back thereof to amorphize the entire silicon thin film placed over the back, thereby forming a light-penetration preventing film.
As disclosed in, for example, the patent document 2, however, the conventional manufacturing method of semiconductor device needs to form a silicon oxide film over the silicon thin film placed over the surface of the sapphire substrate for the purpose of protection of the silicon thin film placed over the surface thereof before the implantation of the ions in the silicon thin film placed over the back thereof, and further to remove the silicon oxide film for the purpose of circuit formation after the ion implantation. It is also necessary to remove the light-penetration preventing film placed over the back after the completion of formation of the semiconductor device with respect to the silicon thin film placed over the surface. Thus, complex process steps were needed to form the light-penetration preventing film over the back.